Influence of water flow velocity, water depth and colony distance on distribution and foraging patterns of terns in the Wadden Sea

Surface-feeding seabirds, such as Common ( Sterna hirundo ) and Arctic Terns ( Sterna paradisaea ) in the German Wadden Sea, are dependent on biological and physical processes that affect prey availability close to the water surface. We proposed the following four hypotheses: (i) relationships should exist between high water flow velocity and foraging activity of terns, as turbulence should enhance prey availability at the surface; (ii) the areas of highest foraging success should be located within areas of low water depth, due to enhanced biological productivity; (iii) as terns are known to have small foraging radii, the location of their breeding grounds should be related to the location of their foraging grounds; (iv) terns should forage intensely in river estuaries, as these should hold ample food supplies. The time between terns leaving the colony and their first foraging attempt differed significantly among different tidal stages: the time was shortest during flood and ebb tides (i.e., highest water flow velocities). Modelling of a long-term data set revealed the highest probability of foraging activity in conditions of high water currents, in both shallow areas and in areas of around 15–20 m depth. Foraging activity was negatively correlated with distance from colony. The distance to the closest estuary had no significant effect on foraging behaviour. Our findings emphasize the physical–biological coupling in the Wadden Sea and highlight the overall importance of small-scale physical processes in directly influencing prey availability for surface-feeding seabirds.